Photon-measurement density function(PMDF),which is the keel of fluorescence molecular tomography(FMT),largely determines the accuracy of reconstruction result of FMT.Based on the direct method,we propose an expression of PMDF in FMT,which is derived from the finite dement method(FEM)solution of the diffusion equation.Compared with the traditional expression based on the perturbation method,the accuracy of expression based on the direct method is shown in theory.Lastly the reconstruction results of phantoms prove this accuracy in experiment.